Overview

As part of a project I’m working on, I wanted to be able to transmit data over a long range laser module. A common green laser pointer works great for this purpose, because they work by doubling the frequency of an infrared laser diode (Diode-Pumped Solid-State Laser)


First the 808nm beam pumps a Nd:YVO4 crystal to create 1064nm light, which then passes through a KTP crystal to double it to 532nm light. This process is explained in detail on wikipedia: DPSS Laser Module

However, the crystals are only about 20% efficient in converting the IR light to green light, thus an IR filter is needed to remove the harmful invisible radiation.

IR Communication

How does this help us to send information? 

Well a common TV remote uses an IR LED and an IR Receiver chip to send data. It is sent using a 45kHz carrier signal to ensure it doesn’t pick up random background IR light (such as from the sun).

The question is, can an IR laser diode be used in place of an IR LED?

Hacking the Module

Forewarning: IR laser light is dangerous! Just because you can’t see it doesn’t mean it isn’t burning a hole in your retina. Even shining it at a surface can be harmful!

To determine if the laser diode could be switched on and off fast enough, I modified the circuit so I could modulate the laser light with a digital signal.

DPSS lasers usually have some sort of driving circuit. In my case, it was a simple op-amp with a constant current feedback path. By grounding the -ve opamp input, I could turn the laser off.

– schematic –

Testing

I set up two arduinos, one connected to the laser module, the other connected to an IR receiver.

Here is the waveform going into the laser (TXD) and coming out of the IR receiver (RXD):

ir_uart

Success!

An interesting side effect, is that when driving the laser at 45kHz, the green light completely disappears, while the IR light remains. The reason behind this is because the pump crystal takes time to “warm up”, and at 45kHz it doesn’t get enough time.

This sort of defeats the purpose of using a green laser. Having visible green laser light should make it safer, as the human eye can see green light and the iris will contract